Mid-infrared optical fibers with enhanced oh-diffusion resistance

ABSTRACT

Mid-infrared-transparent optical fiber products with enhanced resistance to OH diffusion are disclosed, which may be used fiber laser oscillator and amplifiers systems. In one embodiment, an optical fiber product may include optical fiber configured for propagation of mid-infrared radiation toward a light-radiating endface of or coupled to the optical fiber, and a diffusion barrier disposed on the light-radiating endface and configured for allowing the mid-infrared radiation emanating from the light-radiating endface to pass therethrough and for preventing OH diffusion therethrough toward the light-radiating endface. In another embodiment, an optical fiber product may include an optical fiber for propagation of mid-infrared radiation and an endcap coupled to the optical fiber for receiving therefrom the mid-infrared radiation and radiating out the mid-infrared radiation, the endcap being made of an endcap material that has no or a low amount of fluoride and that is less permeable to OH diffusion than the fiber-optic material.

RELATED PATENT APPLICATION

The present application claims priority to U.S. Provisional PatentApplication No. 62/947,192 filed on Dec. 12, 2019, the disclosure ofwhich is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The technical field generally relates to mid-infrared optical fibers,and more particularly to mid-infrared optical fibers with enhancedresistance to OH diffusion.

BACKGROUND

Fiber laser technologies are playing an instrumental role in thedevelopment of various applications [1]. However, as the output power offiber laser systems increases, the likelihood of endface damageincreases accordingly. In the case of conventional Yb³⁺:silica fiberlasers operating at 1 μm, such failure is generally related to the factthat their output intensity exceeds the air-glass surface damagethreshold, and damage occurs either due to overheating incontinuous-wave (CW) regime or laser-induced breakdown due to intensepulses [2]. To mitigate this issue, fiber-based endcaps, spliced at theoutput of fiber laser systems, have been developed to allow the beam toexpand in a controlled manner and therefore lower its intensity belowthe glass damage threshold. Such endcaps have enabled the demonstrationof fiber laser systems delivering over 100 kW of output power in CWoperation [3].

Fluoride-based fiber lasers provide the means to achieve powerful laseremission between 2.8 and 4 μm [4-7], although their current output poweris noticeably less than that of their silica-based counterparts. Yet, anerbium-doped zirconium fluoride fiber laser delivering 42 W of CW outputpower at 2.83 μm was recently reported, which highlighted the potentialof 3-μm fiber lasers for further power-scaling up to 100-W levels [4].Such all-fiber laser sources are coveted in the development ofbiological tissue ablation and remote-sensing applications, given theirexcellent overlap with the strong vibrational absorption band of OHbonds, their high beam quality, and their compact yet rugged design [8].

However, the widespread deployment of high-power 3-μm-class all-fiberlasers has been hindered by the short lifetime of such laser sources dueto fiber tip degradation through moisture diffusion. This phenomenon isspecific to these lasers, due to the overlap between their emissionspectrum and the strong OH absorption band around 3 μm and thehygroscopic nature of fluoride-based glasses. Specifically, when the tipof a fluoride glass fiber is exposed to ambient air, water vapor canreact with the glass constituents, which tend to increase theconcentration of OH compounds at the fiber tip. These OH compounds maybe absorbed by the glass structure, where they can diffuse according toFick's laws of diffusion. Due to the strong absorption of laserradiation at around 3 μm by OH compounds, laser absorption increases asthe number of OH compounds increase, which causes local heating at thefiber tip and, in turn, leads to a concomitant enhancement of thediffusion process, and so forth. This positive feedback loop mayultimately lead to the catastrophic destruction of the fiber tip.Through analytical modeling, it was demonstrated that the time elapsedbefore catastrophic failure of a fluoride fiber tip was inverselyproportional to the square of the 3-μm output power [9]. As a result, ata 20-W power level, the all-fiber cavity reported in [4] lasted lessthan 10 hours before the fluoride-based endcap of the fiber laserunderwent catastrophic failure due to OH diffusion.

Thus, there remains a need for mid-infrared optical fibers with enhancedresistance to OH diffusion.

SUMMARY

The present description generally relates to mid-infrared-transparentoptical fibers having an enhanced resistance to OH diffusion, as can becaused by the in-diffusion of water vapor from the ambient environment.The optical fibers disclosed herein may be used, for example, inmid-infrared fiber lasers operating around 3 μm to mitigate or suppressOH-diffusion-induced fiber tip degradation.

In accordance with an aspect, there is provided an optical fiberproduct. The optical fiber product includes: an optical fiber made of afiber-optic material and configured for propagation of mid-infraredradiation toward a light-radiating endface of or coupled to the opticalfiber; and a diffusion barrier disposed on the light-radiating endfaceand configured for allowing the mid-infrared radiation emanating fromthe light-radiating endface to pass therethrough and for preventing OHdiffusion therethrough toward the light-radiating endface.

In one embodiment, the fiber-optic material includes a glass material.For example, the glass material may include a fluoride-based glass, achalcogenide-based glass, a chalcohalide-based glass, an oxide-basedglass, a tellurite-based glass, or any combination thereof. In anotherembodiment, the fiber-optic material includes a crystal material.

In one embodiment, the light-radiating endface is an endface of theoptical fiber. In another embodiment, the light-radiating endface is anendface of an endcap coupled between the optical fiber and the diffusionbarrier. In one embodiment, the endcap is made of an endcap materialthat is less permeable to OH diffusion than the fiber-optic material.

In one embodiment, the light-radiating endface is perpendicular to alongitudinal fiber axis of the optical fiber product. In anotherembodiment, the light-radiating endface is oblique to a longitudinalfiber axis of the optical fiber product.

In one embodiment, the diffusion barrier includes a thin-film coating.Depending on the application, the thin-film coating may include a singlethin-film layer or a multiple thin-film layers.

In one embodiment, the diffusion barrier extends over all of thelight-radiating endface.

In one embodiment, the diffusion barrier has a thickness ranging fromabout 1 nm to about 10 μm, for example, from about 10 nm and about 100nm.

In one embodiment, the diffusion barrier is made of a barrier materialincluding a dielectric, a carbon-based material, a metal, a metalloid, ametal oxide, an alloy, a composite material, or any combination thereof.In one embodiment, the diffusion barrier is made of a barrier materialincluding a ceramic compound, for example, a carbide, a nitride, aboride, an oxide, or any combination thereof. In one embodiment, theceramic compound includes silicon nitride, silicon oxynitride, siliconcarbide, boron nitride, silicon carbide, boron carbide, tungstencarbide, or any combination thereof.

In one embodiment, the optical fiber is made of a fluoride glass, forexample, a fluorozirconate glass including ZrF₄ as a major component,such as ZBLAN (ZrF₄—BaF₂—LaF₃—AlF₃—NaF), or a fluoroaluminate glassincluding AlF₃ as a major component, or a fluoroindate glass includingInF₃ as a major component, or a fluorophosphate glass includingP₂O₅—AlF₃ as a major component, or any combination thereof.

In one embodiment, the optical fiber is made of a chalcogenide glass,for example, a sulfide glass including As₂S₃ as a major component, or aselenide glass including As₂Se₃ as a major component, or a tellurideglass including GeTe as a major component, or a mixture thereof forminga multi-material glass, such as GeAsTeSe, or any combination thereof.

In another embodiment, the optical fiber is made of an oxide glass, forexample, a germanium-oxide glass including GeO₂ as major component, or alead-germanate glass including GeO₂—PbO as a major component, or aphosphate glass including P₂O₅ as major component, or a BGG glassincluding BaO—Ga₂O₃—GeO₂ as a major component, or any combinationthereof.

In another embodiment, the optical fiber is made of a crystal materialcapable of transmitting mid-infrared radiation. Non-limiting examples ofcrystal materials that can support mid-infrared transmission includemonocrystalline materials, such as single-crystal sapphire (Al₂O₃) andyttrium aluminum garnet (YAG; Y₃Al₅O₁₂), and polycrystalline materials,such as halide materials, for example, silver halides.

In one embodiment, the light-radiating endface is an endface of theoptical fiber. However, in another embodiment, the light-radiatingendface is an endface of an endcap connected to the optical fiber. Inone embodiment, the endcap is made of an endcap material that is lesspermeable to OH diffusion than the mid-infrared-transparent material ofthe optical fiber.

In one embodiment, the diffusion barrier is a thin-film coating having athickness sufficient to impart OH-diffusion resistance to the opticalfiber product. For example, the thickness of the thin-film coating canrange from about 1 nm to about 10 μm, depending on the endcapcomposition, particularly between 10 nm and 100 nm. It is appreciatedthat various types of mid-infrared-transparent andOH-diffusion-resistant barrier materials can be used to form thediffusion barrier. Non-limiting examples include dielectrics (e.g.,crystals, glasses, ceramics, and polymers); carbon-based materials, suchas diamond; metals, such as gold, aluminum, tantalum, titanium, andcobalt; metalloids, such as boron, silicon, and germanium; metal oxides,such as Si—TiN—O; alloys; composite materials; and mixtures thereof. Inone embodiment, the barrier material is a ceramic compound, such as acarbide, a nitride, a boride, an oxide, or a mixture thereof.Non-limiting examples of such ceramic compounds include silicon nitride,silicon oxynitride, silicon carbide, boron nitride, silicon carbide,boron carbide, tungsten carbide, and the like.

In accordance with another aspect, there is provided an optical fiberproduct. The optical fiber product includes: an optical fiber made of afiber-optic material and configured for propagation of mid-infraredradiation; and an endcap having a proximal endface coupled to theoptical fiber for receiving therefrom the mid-infrared radiation, adistal endface for radiating the mid-infrared radiation outside theoptical fiber product, and an endcap body extending and configured forpropagation of the mid-infrared radiation from the proximal endface tothe distal endface, the endcap being made of an endcap material that hasno or a low amount of fluoride and that is less permeable to OHdiffusion than the fiber-optic material.

In one embodiment, the fiber-optic material includes a glass material.For example, the glass material may include a fluoride-based glass, achalcogenide-based glass, a chalcohalide-based glass, an oxide-basedglass, a tellurite-based glass, or any combination thereof. In anotherembodiment, the fiber-optic material includes a crystal material.

In one embodiment, the endcap material includes a glass material. Forexample, the glass material may include an oxide-based glass, such assilica, an aluminosilicate-based glass, a phosphosilicate-based glass,an aluminophosphosilicate-based glass, a germanium-oxide-based glass, alead-germanate-based glass, a tellurium-oxide-based glass, or anycombination thereof. As another example, the glass material may includea barium gallium germanate glass, a tellurite-based glass, achalcogenide-based glass, or any combination thereof.

In one embodiment, endcap material includes a crystal material. Forexample, the crystal material may include an oxide-based crystalmaterial, such as sapphire, a garnet crystal material, or a combinationthereof.

In one embodiment, the endcap material has a molar proportion offluoride or fluoride-based compounds that is less than 20 mol %, or lessthan 10 mol %. In one embodiment, the endcap material contains no ortrace amounts of fluoride or fluoride-based compounds.

In one embodiment, the endcap has a length ranging from about 50 μm toabout 6 mm.

In one embodiment, the endcap has a coreless structure. In anotherembodiment, the endcap has a core-clad structure.

In one embodiment, the distal endface of the endcap is perpendicular toa longitudinal fiber axis of the optical fiber product. In anotherembodiment, the distal endface of the endcap is oblique to alongitudinal fiber axis of the optical fiber product.

In one embodiment, the optical fiber is made of a fluoride glass, forexample, a fluorozirconate glass including ZrF₄ as a major component,such as ZBLAN (ZrF₄—BaF₂—LaF₃—AlF₃—NaF), or a fluoroaluminate glassincluding AlF₃ as a major component, or a fluoroindate glass includingInF₃ as a major component, or a fluorophosphate glass includingP₂O₅—AlF₃ as a major component, or any combination thereof.

In another embodiment, the optical fiber is made of a chalcogenideglass, for example, a sulfide glass including As₂S₃ as a majorcomponent, or a selenide glass including As₂Se₃ as a major component, ora telluride glass including GeTe as a major component, or a mixturethereof to form a multi-material glass such as GeAsTeSe, or anycombination thereof.

In another embodiment, the optical fiber is made of an oxide glass, forexample, a germanium-oxide glass including GeO₂ as major component, or alead-germanate glass including GeO₂—PbO as a major component, orphosphate glass including P₂O₅ as major component, or BGG glassincluding BaO—Ga₂O₃—GeO₂ as a major component, or any combinationthereof.

In another embodiment, the optical fiber is made of a crystal materialcapable of transmitting mid-infrared radiation. Non-limiting examples ofcrystal materials that can support mid-infrared transmission includemonocrystalline materials, such as single-crystal sapphire (Al₂O₃) oryttrium aluminum garnet (YAG; Y₃Al₅O₁₂), and polycrystalline materials,such as halide materials, for example, silver halides.

In one embodiment, the endcap material is made of an oxide-based glassmaterial, such as silica (SiO₂), an aluminosilicate-based glass, aphosphosilicate-based glass material, an aluminophosphosilicate-basedglass material, a germanium-oxide-based glass material, alead-germanate-based glass material, and a tellurium-oxide-based glassmaterial. In another embodiment, the endcap material is made of anothertype of non-fluoride-based glass material, such as a BGG glass material,a tellurite-based glass material, and a chalcogenide-based glassmaterial. In another embodiment, the endcap material can be anon-glass-based material, non-limiting examples of which includeoxide-based crystal materials, such as sapphire (Al₂O₃), yttriumaluminum garnet (YAG; Y₃Al₅O₁₂), and other types of garnet crystalmaterials; and other types of crystal materials, such as zinc selenide(ZnSe), and diamond.

In accordance with another aspect, there is provided a fiber-based laseroscillator, laser amplifier, or laser beam delivery system including anoptical fiber product as disclosed herein. In such embodiments, theoptical fiber can include a gain region, for example, a laser-activeregion (e.g., a rare-earth doped active region) or a non-linear gainregion, that defines a gain medium that can be stimulated to emit laserradiation by optical pumping.

Other objects, features and advantages of the present description willbecome more apparent upon reading of the following non-restrictivedescription of specific embodiments thereof, given by way of exampleonly with reference to the appended drawings. Although specific featuresdescribed in the above summary and in the detailed description below maybe described with respect to specific embodiments or aspects, it shouldbe noted that these specific features can be combined with one anotherunless stated otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an optical fiber product, inaccordance with a possible embodiment.

FIG. 2 is a schematic representation of an optical fiber product, inaccordance with another possible embodiment.

FIG. 3 is a schematic representation of an optical fiber product, inaccordance with another possible embodiment.

FIG. 4 is a schematic representation of an optical fiber product, inaccordance with another possible embodiment.

FIG. 5 is a schematic representation of an optical fiber product, inaccordance with another possible embodiment.

FIG. 6 is a schematic representation of an optical fiber product, inaccordance with another possible embodiment.

FIG. 7 is a schematic representation of an optical fiber product, inaccordance with another possible embodiment.

FIG. 8 is a schematic representation of an optical fiber product, inaccordance with another possible embodiment.

FIG. 9 is a schematic representation of an optical fiber product, inaccordance with another possible embodiment.

FIG. 10 is a schematic representation of an experimental setup used in astudy to monitor the degradation over time of different endcapssubjected to 20 W of output power at 3 μm.

FIG. 11 is a schematic representation of a laser beam delivery systemincluding an optical fiber product.

FIGS. 12(a) to 12(f) are photographs of fiber endcaps tested in thestudy.

FIGS. 13(a) and 13(b) are photographs of the interface between a ZrF₄relay fiber and an SiO₂ endcap and the interface between a ZrF₄ relayfiber and an Er³⁺:YAG endcap, respectively.

FIG. 14 depicts time-dependent fiber endcap temperature measurements ata constant 3-μm output power of 20 W. Each curve in FIG. 14 depicts themeasured temperature of a respective endcap as a function of time over aperiod of 100 hours.

FIGS. 15(a) to 15(c) depict time-dependent fiber endcap temperaturemeasurements performed on Si₃N₄-coated and uncoated fiber endcaps over aperiod of 100 hours. FIG. 15(a): comparison between Si₃N₄-coated anduncoated ZrF₄ endcaps (Si₃N₄ coating thickness: 25 nm; 3-μm outputpower: 7 W). FIG. 15(b): comparison between Si₃N₄-coated and uncoatedAlF₃ endcaps (Si₃N₄ coating thickness: 25 nm; 3-μm output power: 7 W).FIG. 15(c): comparison between Si₃N₄-coated and uncoated Al₂O₃ endcaps(Si₃N₄ coating thickness: 100 nm; 3-μm output power: 20 W).

DETAILED DESCRIPTION

In the present description, similar features in the drawings have beengiven similar reference numerals. To avoid cluttering certain figures,some elements may not be indicated if they were already identified in apreceding figure. It is appreciated that the elements of the drawingsare not necessarily depicted to scale since emphasis is placed onclearly illustrating the elements and structures of the presentembodiments. Furthermore, positional descriptors indicating the locationand/or orientation of one element with respect to another element areused herein for ease and clarity of description. Unless otherwiseindicated, these positional descriptors should be taken in the contextof the figures and should not be considered limiting. It is appreciatedthat such spatially relative terms are intended to encompass differentorientations in the use or operation of the present embodiments, inaddition to the orientations exemplified in the figures. Furthermore,when a first element is referred to as being “on”, “above”, “below”,“over”, or “under” a second element, the first element can be eitherdirectly or indirectly on, above, below, over, or under the secondelement, respectively, such that one or multiple intervening elementsmay be disposed between the first element and the second element.

The terms “a”, “an” and “one” are defined herein to mean “at least one”,that is, these terms do not exclude a plural number of items unlessstated otherwise.

Terms such as “substantially”, “generally”, and “about”, which modify avalue, condition, or characteristic of a feature of an exemplaryembodiment, should be understood to mean that the value, condition, orcharacteristic is defined within tolerances that are acceptable for theproper operation of this exemplary embodiment for its intendedapplication or that fall within an acceptable range of experimentalerror. In particular, the term “about” generally refers to a range ofnumbers that one skilled in the art would consider equivalent to thestated value (e.g., having the same or equivalent function or result).In some instances, the term “about” means a variation of ±10 percent ofthe stated value. It is noted that all numeric values used herein areassumed to be modified by the term “about”, unless stated otherwise.

The terms “match”, “matching” and “matched” are intended to refer hereinto a condition in which two elements are either the same or within somepredetermined tolerance of each other. That is, these terms are meant toencompass not only “exactly” or “identically” matching the two elements,but also “substantially”, “approximately”, or “subjectively” matchingthe two elements, as well as providing a higher or best match among aplurality of matching possibilities.

The terms “connected” and “coupled”, and derivatives and variantsthereof, are intended to refer herein to any connection or coupling,either direct or indirect, between two or more elements, unless statedotherwise. The connection or coupling between the elements may be, forexample, mechanical, optical, electrical, thermal, magnetic, chemical,fluidic, logical, operational, or any combination thereof.

The present description generally relates to mid-infrared optical fiberproducts with enhanced resistance to OH diffusion, as can be caused whenambient water vapor reacts with an exposed light-radiating fiber endfaceto induce fiber tip degradation or failure.

In accordance with an aspect, the disclosed optical fiber product mayinclude an optical fiber and a diffusion barrier. The optical fiber maybe made of a mid-infrared-transparent material and be configured forguided propagation of mid-infrared radiation therein toward alight-radiating endface. Depending on the application, thelight-radiating endface may be an endface of the optical fiber itself orthe endface of a fiber endcap spliced, fused, or otherwise connected orcoupled to the optical fiber. The diffusion barrier may be deposited,formed, or otherwise provided on the light-radiating endface andconfigured for allowing the transmission of the mid-infrared radiationemanating from the light-radiating endface and for preventing OHdiffusion through to the light-radiating endface. When the optical fiberis terminated with a fiber endcap, the endcap may be made of an endcapmaterial that is less permeable to OH diffusion than themid-infrared-transparent material of the optical fiber.

In the present description, the terms “prevent” and “preventing”, andother variants and derivatives thereof, are used in a broad sense todescribe a diffusion barrier that substantially or completely restrictsOH diffusion therethrough. Thus, the terms “prevent” and “preventing”are intended to encompass reducing, decreasing, inhibiting, impeding,hindering, mitigating, stopping, and/or eliminating OH diffusion throughthe diffusion barrier. For greater clarity, the terms “prevent” and“preventing” do not necessarily mean that, in an exemplary embodiment,there is absolutely no OH diffusion occurring through the diffusionbarrier, but rather that OH diffusion through the diffusion barrier isrestricted or reduced to a level that is a sufficiently low for theproper operation of this exemplary embodiment for its intendedapplication.

In accordance with another aspect, the disclosed optical fiber productmay include an optical fiber terminated with a fiber endcap. The opticalfiber may be made of a mid-infrared-transparent fiber-optic material andbe configured for guided propagation of mid-infrared radiation therein.The fiber endcap may define an optical medium or material extendingbetween a proximal endface, coupled to the optical fiber for receivingtherefrom the mid-infrared radiation, and a distal endface, defining alight-radiating endface for radiating the mid-infrared radiation outsidethe optical fiber product. The fiber endcap may be made of an endcapmaterial that has no or a low amount of fluoride and that is lesspermeable to OH diffusion than the mid-infrared-transparent fiber-opticmaterial.

The present techniques may be used or implemented in variousapplications that may require or benefit from mid-infrared opticalfibers with enhanced resistance to OH diffusion and concomitantmitigation of fiber tip degradation, which may provide improved fiberperformance in terms of lifetime and output power. The techniquesdisclosed herein may be applied to or implemented in various types ofmid-infrared fiber laser oscillator and amplifier systems as well as inother optical systems that may use or rely on mid-infrared fibers, suchas optical parametric oscillators (OPOs). Such fiber laser oscillatorand amplifier systems typically involve double-clad rare-earth-dopedfluoride fibers that are optically pumped (e.g., by a diode pump source)to generate mid-infrared laser emission around 3 μm. Non-limitingexamples of fields where the present techniques may be used include, toname a few, materials processing, medicine and surgery, metrology,spectroscopy, sensing and imaging, security and defense,telecommunications, and countermeasure applications.

In the present description, the terms “light” and “optical”, andvariants and derivatives thereof, are intended to refer to radiation inany appropriate region of the electromagnetic spectrum, and are notlimited to visible light. By way of example, in some embodiments, theterms “light” and “optical” may encompass infrared radiation,particularly mid-infrared radiation. Infrared radiation is commonlydivided into the near-infrared region for wavelengths ranging from about0.7 to 2.5 μm; the mid-infrared region for wavelengths ranging fromabout 2.5 to 25 μm; and the far-infrared region for wavelengths aboveabout 25 μm. It is appreciated that the definitions of differentinfrared regions in terms of spectral ranges, as well as their limits,may vary depending on the technical field under consideration, and arenot meant to limit the scope of application of the present techniques.In particular, the term “mid-infrared” is used throughout the presentdescription. The definition of the term “mid-infrared” remains somewhatunsettled in the art, with the boundaries between near-infrared,mid-infrared, and far-infrared regions varying in different technicalfields. As used herein, the term “mid-infrared” is intended to refer tothe region of the electromagnetic spectrum encompassing at leastwavelengths ranging from about 2.5 μm to about 25 μm. In certaintechnical fields, the boundary between mid-infrared and near-infraredmay correspond to wavelengths as long as 50 μm, while the boundarybetween mid-infrared and far-infrared may correspond to wavelengths asshort as 1.5 μm.

The optical fibers disclosed herein may be a made from a variety ofmid-infrared-transparent materials, whether glassy or crystalline, thatcan allow guided propagation of one or more modes at mid-infraredwavelengths.

In some embodiments, the mid-infrared-transparent fiber-optic materialmay be a glass material having a transmittance window in themid-infrared. Non-limiting examples of classes of possible glassmaterials include fluoride-based glasses; chalcogenide-based glasses;chalcohalide-based glasses; oxide-based glasses; tellurite-basedglasses; and other glass materials with similar physical properties, andany combination thereof. Depending on the application, the glassmaterial may be doped or undoped.

In some embodiments, the mid-infrared-transparent fiber-optic materialmay be a fluoride-based glass. For example, the fluoride-based glass mayinclude a fluorozirconate glass having a composition including ZrF₄,such as a ZBLA glass, a ZBLAN glass, and a ZBLALi glass; or afluoroaluminate glass having a composition including AlF₃; or afluoroindate glass having a composition including InF₃; or afluorophosphate glass having a composition including P₂O₅—AlF₃; afluoro-sulfo-phosphate glass; or any combination thereof.

In some embodiments, the mid-infrared-transparent fiber-optic materialmay be a chalcogenide-based glass having a composition including As₂S₃,As₂Se₃, As₂Te₃, AsSSe, AsSTe, GaLaS, GeTe, GeAsS, GeAsSe, or anycombination thereof. For example, the chalcogenide-based glass mayinclude a sulfide glass including As₂S₃ as a major component; a selenideglass including As₂Se₃ as a major component; a telluride glass includingGeTe as a major component; a mixture thereof forming a multi-materialglass, such as GeAsTeSe; or any combination thereof.

In some embodiments, the mid-infrared-transparent fiber material may bean oxide-based glass, for example a germanium-oxide glass having acomposition including GeO₂; a lead-germanate glass having a compositionincluding GeO₂—PbO; a phosphate glass having a composition includingP₂O₅; a barium gallium germanate (BGG) glass having a compositionincluding BaO—Ga₂O₃—GeO₂; or any combination thereof.

In some applications, several mid-infrared-transparent glassmaterials—including various fluoride-based, chalcogenide-based,chalcohalide-based glasses, and tellurite-based glasses such as thosementioned above—may be referred to as “low phonon energy glasses”. Inthe present description, the term “low phonon energy glass” is intendedto refer to any glass having a maximum phonon energy lower than thephonon energy of silica-based glass, that is, lower than about 1100cm⁻¹. Optical fibers made of a low phonon energy glass generally have atransmittance window extending in the mid-infrared (e.g., from about 2.5μm to longer wavelengths).

In some embodiments, the mid-infrared-transparent fiber-optic materialmay not be a glass material. For example, the optical fiber may be madeof a crystal material capable of transmitting mid-infrared radiation.Non-limiting examples of crystal materials that can support mid-infraredtransmission include monocrystalline materials, such as single-crystalsapphire (Al₂O₃) or yttrium aluminum garnet (YAG; Y₃Al₅O₁₂), or anycombination thereof, and polycrystalline materials, such as halidematerials, for example silver halides.

Various implementations of the present techniques are described belowwith reference to the figures.

Referring to FIG. 1, there is depicted a schematic representation of anoptical fiber product 100, in accordance with a possible embodiment. Theoptical fiber product 100 generally includes an optical fiber 102 and adiffusion barrier 104. The optical fiber 102 is made of amid-infrared-transparent fiber-optic material, such as those mentionedabove. The optical fiber 102 is configured for supporting guidedpropagation of mid-infrared radiation 106 therein toward alight-radiating endface 108. In the embodiment of FIG. 1, thelight-radiating endface 108 is an endface of the optical fiber 102itself. However, in other embodiments, such as the one depicted in FIG.2, the light-radiating endface 108 may be an endface of a fiber endcap110 serially connected to the optical fiber 102. Depending on theapplication, the light-radiating endface 108 may be perpendicular to alongitudinal fiber axis 122 of the optical fiber product 100, as inFIGS. 1 and 2, or oblique to the longitudinal fiber axis 122, as inFIGS. 3 and 4. The diffusion barrier 104 is disposed on thelight-radiating endface 108. The diffusion barrier 104 is configured fortransmitting therethrough the mid-infrared radiation 106 emanating fromthe light-radiating endface 108. The diffusion barrier 104 is alsoconfigured for preventing OH diffusion therethrough toward thelight-radiating endface 108. The composition, structure, configuration,and operation of these and other possible components of the opticalfiber product 100 will be described in greater detail below.

It is appreciated that the optical fiber product 100 of FIG. 1 may beprovided as a component of a mid-infrared fiber laser oscillator oramplifier system 200, such as the one described below with reference toFIG. 10, or another system, for example, as a laser light delivery cablein laser beam delivery system 300, as depicted schematically in FIG. 11.It is also appreciated that in fiber laser systems, the optical fiber102 may include a gain region that can be stimulated by optical pumpingto emit mid-infrared radiation 106; undoped, passive regions topropagate light from and to the active region; reflectors, such as Bragggratings, provided on either side of the gain region; a pump stripper toremove residual pump power; splice junctions; and the like. However, theembodiment of FIG. 1—as well as those of FIGS. 2 to 9—depicts only adownstream portion of the optical fiber product 100, which includes thelight-radiating endface 108 through which the mid-infrared radiation 106is radiated out, for example, as laser radiation. It is appreciated thatthe general principles underlying the configuration, operation, andapplications of mid-infrared fiber laser oscillators and amplifiers areknown in the art and need not be described in greater detail herein.Examples of mid-infrared fiber laser oscillators and amplifiers aredescribed in the following co-assigned patent documents, the entirecontents of which are incorporated herein by reference: U.S. Pat. No.10,084,287 B2 and Intl Pat. Appl. Pub. No. WO 2011/009198 A1.

In FIG. 1, the optical fiber 102 includes a core 112 and a cladding 114surrounding the core 112. A protective polymer coating (not shown) maybe disposed around the cladding 114. The core 112 forms a light-guidingpath along which the mid-infrared radiation 106 is guided. The core 112is made of a core material having a refractive index higher than therefractive index of the cladding material so that light can be guidedtherealong by total internal reflection at the interface between thecore 112 and the cladding 114, as depicted schematically in FIG. 1.Depending on the application, the core 112 may be single mode ormultimode, centered or off-centered relative to the fiber axis 122, andconfigured to support different polarization states. In someimplementations, multicore optical fibers may be used. The core 112 andthe cladding 114 may have various compositions and refractive indexprofiles (e.g., graded-index profile or step-index profile). In oneembodiment, the core 112 and the cladding 114 may be made of amid-infrared-transparent glass material, such as a fluoride glass, withthe core material having a refractive index higher than that of thecladding material. For example, the core 112 may contain one or moreindex-changing dopants to raise its refractive index relative to that ofthe cladding 114. The core 112 may have a diameter ranging from about 3μm to about 450 μm, while the cladding 114 may have a diameter rangingfrom about 80 μm to about 500 μm, although other core and cladding sizesmay be used in other embodiments. Depending on the application, the core112 and the cladding 114 may each have a circular or a noncircularcross-section.

The cladding 114 may include one or more cladding layers. Fiber laserscommonly use double-clad optical fibers, which include a core carryingthe laser signal, an inner pump cladding surrounding the core andcarrying the pump signal, and an outer cladding surrounding the pumpcladding. In such applications, the core, inner pump cladding, and outercladding are made of materials with different refractive indices,selected so that the laser signal and the pump signal are guided bytotal internal reflection inside the core and inside the pump cladding,respectively. This is achieved with the pump-cladding index being lowerthan the core index and higher than the outer-cladding index.

Depending on the application, the optical fiber 102 may be a passivefiber or an active fiber including a gain medium for providing opticalamplification. The gain medium may be doped with rare-earth elements(e.g., ytterbium, erbium, holmium, thulium, praseodymium, neodymium,dysprosium, and the like, and combinations thereof) or other dopants, asthe case may be. As noted above, when used in fiber lasers, the opticalfiber 102 may include both active and passive segments along its length.

It is appreciated that, in general, the composition, cross-sectionalshape and size, refractive index profile, number of cores, number ofguided modes, passive or active operation mode, operating wavelengthrange, polarization-maintaining (PM) properties, and other core,cladding, and fiber properties and characteristics may be varied inaccordance with a specified application.

Referring still to FIG. 1, the diffusion barrier 104 disposed on thelight-radiating endface 108 is configured for preventing OH compounds(e.g., contaminants and impurities containing hydroxide ions and/orhydroxy groups) from diffusing therethrough and reaching—and potentiallydegrading—the optical fiber 102. It has been found that without theprovision of the diffusion barrier 104, the light-radiating endface 108is exposed to the surrounding environment such that ambient water vapor(or liquid water, in some instances) may interact with the glassconstituents of the optical fiber 102 and increase the concentration ofOH compounds on its exposed endface 108. Since manymid-infrared-transparent glasses, including ZrF₄-based fluoride glasses,have a relatively high OH permeability, these OH compounds tend todiffuse inside the optical fiber 102. The OH compounds having diffusedinside the optical fiber 102 may absorb part of the mid-infraredradiation 106 exiting the optical fiber 102 at the light-radiatingendface 108, due to the strong mid-infrared OH absorption region near 3μm. The absorbed radiation may heat up the light-radiating endface 108,which in turn may enhance the OH diffusion process, and so forth. Thiscascading feedback loop may gradually degrade the endface quality of theoptical fiber 102 and eventually cause its failure after a certainperiod of operation.

In one embodiment, the diffusion barrier 104 may be a thin-film coatingformed on the light-radiating endface 108 and having a thicknesssufficient to impart OH-diffusion resistance to the optical fiberproduct 100. For example, depending on the endcap composition, thethickness of the thin-film coating can range from about 1 nm to about 10μm, particularly between 10 nm and 100 nm. In general, the thickness ofthe diffusion barrier 104 may be adjusted to ensure or help ensure OHimpermeability, mid-infrared transparency, and mechanical integrity.Depending on the application, the thin-film coating forming thediffusion barrier 104 can include a single thin-film layer, as in FIGS.1 to 4 and 6, or multiple thin-film layers 124 a, 124 b, 124 c, as inFIG. 5. Furthermore, the diffusion barrier 104 may extend over part orall of the light-radiating endface 108.

It is appreciated that various types of mid-infrared-transparent andOH-diffusion-resistant barrier materials may be used as the diffusionbarrier 104. Non-limiting examples include dielectrics (e.g., crystals,glasses, ceramics, and polymers); carbon-based materials, such asdiamond; metals, such as gold, aluminum, tantalum, titanium, and cobalt;metalloids, such as boron, silicon, and germanium; metal oxides, such asSi—TiN—O; alloys; composite materials; and mixtures and combinationsthereof. In one embodiment, the barrier material may be a ceramiccompound, such as a carbide, a nitride, a boride, an oxide, or acombination thereof. Non-limiting examples of such ceramic compoundsinclude silicon nitride, silicon oxynitride, silicon carbide, boronnitride, silicon carbide, boron carbide, tungsten carbide, and the like,and any combination thereof. It is appreciated that the choice of asuitable barrier material may be made based on a number of factors,non-limiting examples of which include cost, availability of materialsand deposition techniques, mechanical, thermal, and chemical stability,and compatibility with the fiber-optic material. It is also appreciatedthat the diffusion barrier 104 may be deposited on the light-radiatingendface 10 using a variety of thin-film deposition techniques, includingphysical deposition techniques, such as radio frequency sputtering,reactive alternative current magnetron sputtering, thermal evaporation,and electron beam physical vapor deposition, chemical depositiontechniques, such as plasma-enhanced chemical vapor deposition and lowpressure chemical vapor deposition techniques, or any other appropriatedeposition techniques.

Referring to FIG. 2, there is illustrated another embodiment of anoptical fiber product 100. The embodiment of FIG. 2 shares severalfeatures with the embodiment of FIG. 1, which will not be described indetail again other than to highlight differences between them. Theoptical fiber product 100 of FIG. 2 includes an optical fiber 102 madeof a mid-infrared-transparent material, such as those mentioned above.The optical fiber 102 has a core 112 and a cladding 114 and isconfigured for guided propagation of mid-infrared radiation 106therealong. The optical fiber product 100 also includes alight-radiating endface 108 for outputting the mid-infrared radiation106, and a diffusion barrier 104. However, in contrast to the embodimentof FIG. 1, the optical fiber product 100 further includes a fiber endcap110 spliced, fused, or otherwise connected or coupled to the outputendface 116 of the optical fiber 102. The endcap 110 defines an opticalmedium or material that extends between a proximal endface 118,optically coupled to the output endface 116 of the optical fiber 102 forreceiving therefrom the mid-infrared radiation 106, and a distal endface120, for radiating the mid-infrared radiation 106 out of the opticalfiber product 100. It is appreciated that in the embodiment of FIG. 2,the light-radiating endface 108 of the optical fiber product 100, onwhich is deposited the diffusion barrier 104, corresponds not to theoutput endface 116 of the optical fiber 102, as in FIG. 1, but to thedistal endface 120 of the endcap 110.

The endcap 110 may reduce the optical power density of the beam ofmid-infrared radiation 106 at the light-radiating endface 108 byallowing the beam to expand in a controlled manner prior to exiting theoptical fiber product 100. The provision of the endcap 110 may can beuseful for achieving high-power generation. Furthermore, if the endcap110 is made of an endcap material that is less permeable to OH diffusionthan the mid-infrared-transparent material of the optical fiber 102, theendcap 110 may also provide, in addition to the diffusion barrier 104,an additional protection against OH diffusion into the optical fiber 102and its potential degradation.

In one embodiment, the endcap 110 may have a circular or a noncircularcross-section, a diameter ranging from about 80 μm to about 12.5 mm, forexample, between about 200 and 500 μm, and a length ranging from about50 μm to about 6 mm, for example, between about 200 and about 750 μm,although other diameter and length values can be used in someembodiments. Depending on the application, the cross-sectional size andshape of the endcap 110 may each be the same as, or different from,those of the optical fiber 102. Furthermore, while the endcap 110depicted in FIG. 2 has a coreless structure and its distal endface 120is perpendicular to the longitudinal fiber axis 122, endcaps having anangled or slanted endface and/or a core-clad structure may be used inother embodiments. For example, in FIG. 4, the distal endface 120 isangled relative to the longitudinal fiber axis 122, while in FIG. 6, theendcap 110 has a core-clad structure including an endcap core 126 and anendcap cladding 128 surrounding the endcap core 126. Depending on theapplication, the endcap core 126 and cladding 128 may or may not havethe same size and shape as the fiber core 112 and cladding 114,respectively. It is appreciated that, in general, the endcapcomposition, internal structure (e.g., coreless or core-clad),cross-sectional shape and size, length, longitudinal profile (e.g.,straight, tapered, or widening), modal operation (i.e., single ormultimode), refractive index profile, operating wavelength range,polarization-maintaining (PM) properties, and other endcap propertiesand characteristics may be varied in accordance with a specifiedapplication and may or may not be the same as those of the optical fiber102.

It is noted that various endcap materials may be used. In oneembodiment, the optical fiber 102 may be made of a fluoride-based glass,while the endcap 110 may be made of a fluoride-based glass that is lesspermeable to OH diffusion than the fluoride-based glass of the opticalfiber 102. In another embodiment, the optical fiber 102 may be made of afluoride-based glass, while the endcap 110 may be made of an endcapmaterial, whether glassy or crystalline, that contains no or a lowamount of fluoride so to avoid or mitigate the high OH permeabilityoften associated with fluoride-based fiber-optic materials. In thepresent description, the term “low amount of fluoride” is intended tomean that the molar proportion of fluoride or fluoride-based compoundsin the endcap material is not more than 50 mol % of its totalcomposition, particularly less than 25 mol % of its total composition,and more particularly less than 10 mol % of its total composition. Inmany cases, the endcap material may contain no or very low amounts(e.g., trace or accidental amounts) of fluoride-compounds. As for thebarrier material, the choice of a suitable endcap material may be madebased on a number of factors, non-limiting examples of which includecost, availability of materials and deposition techniques, mechanical,thermal, and chemical stability, and compatibility with the fiber-opticmaterial.

It is appreciated that the various types of possiblemid-infrared-transparent and OH-diffusion-resistant barrier materialsmentioned with respect to the embodiment of FIG. 1 may also be used toform the diffusion barrier 104 in the embodiment of FIG. 2. However, itappreciated that in this case, the choice of a suitable barrier materialwould generally be made based more on its compatibility with the endcapmaterial, rather than the fiber-optic material.

Referring to FIG. 7, there is illustrated another embodiment of anoptical fiber product 100. The embodiment of FIG. 7 shares severalfeatures with the embodiments of FIGS. 1 to 6, which will not bedescribed in detail again other than to highlight differences betweenthem. As in FIGS. 2, 4, and 6, the optical fiber product 100 of FIG. 7includes an optical fiber 102 and a terminating endcap 110 spliced,fused, or otherwise connected or coupled to the output endface 116 ofthe optical fiber 102. The optical fiber 102 includes a core 112 and acladding 114 surrounding the core 112. The optical fiber 102 is made ofa mid-infrared-transparent material, such as a those mentioned above,and is configured for propagation of mid-infrared radiation 106 therein.The endcap 110 defines an optical medium or material that extendsbetween a proximal endface 118 and a distal endface 120. The proximalendface 118 is coupled to the output endface 116 of the optical fiber102 and is configured for receiving therefrom the mid-infrared radiation106. The distal endface 120 defines a light-radiating endface 108 forradiating the mid-infrared radiation 106 outside the optical fiberproduct 100. As in FIG. 2, the endcap 110 in FIG. 7 may reduce theoptical power density at the light-radiating endface 108 by allowing thebeam of mid-infrared radiation 106 to expand in a controlled mannerprior to exiting the optical fiber product 100. The endcap 110 may alsoprovide a barrier against OH diffusion into the optical fiber 102. Asnoted above, the mid-infrared radiation 106 propagating in the opticalfiber may be a mid-infrared laser beam that is generated by opticallypumping a laser-active region of the fiber core 112 and that exits theoptical fiber product 100 through the light-radiating endface 108 of theendcap 110.

In contrast to the embodiments of FIGS. 1 to 6, the embodiment of FIG. 7does not include an OH-diffusion barrier deposited—for example, as athin-film coating—on the light-radiating endface 108 of the opticalfiber product 100. Rather, in the embodiment of FIG. 7, it is the endcap110 that acts primarily, if not only, as a barrier for preventing OHfrom diffusing therethrough to reach and potentially degrade the tip ofthe optical fiber 102. In the illustrated embodiment, the endcap 110 ismade of an endcap material that is less permeable to OH diffusion thanthe mid-infrared-transparent material of the optical fiber and, inparticular, that has no or a low amount of fluoride. As noted above, theterm “low amount of fluoride” is intended to mean that the molarproportion of fluoride or fluoride-based compounds in the endcapmaterial does not exceed more than 50 mol % of its total composition,particularly less than 25 mol % of its total composition, moreparticularly less than 10 mol % of its total composition, and in manycases even significantly less than 10 mol % of its total composition(e.g., no or only trace amounts).

As noted regarding FIG. 2, the endcap 110 in FIG. 7 may have a circularor a noncircular cross-section, a diameter ranging from about 80 μm toabout 12.5 mm, for example, between about 200 and 500 μm, and a lengthranging from about 50 μm to about 6 mm, for example, between about 200and 750 μm, although other diameter and length values can be used insome embodiments. Depending on the application, the cross-sectional sizeand shape of the endcap 110 may each be the same as, or different from,those of the optical fiber 102. Furthermore, while the endcap 110depicted in FIG. 7 has a coreless structure and its distal endface 120is perpendicular to the longitudinal fiber axis 122, endcaps having anangled or slanted endface and/or a core-clad structure may be used inother embodiments. For example, in FIG. 8, the distal endface 120 isangled or oblique relative to the longitudinal fiber axis 122, while inFIG. 9, the endcap 120 has a core-clad structure including an endcapcore 126 and an endcap cladding 128 surrounding the endcap core 126.Depending on the application, the endcap core 126 and cladding 128 mayor may not have the same size and shape as the fiber core 112 andcladding 114, respectively. It is appreciated that, in general, theendcap composition, internal structure (e.g., coreless or core-clad),cross-sectional shape and size, length, longitudinal profile (e.g.,straight, tapered, or widening), modal operation (i.e., single ormultimode), refractive index profile, operating wavelength range,polarization-maintaining (PM) properties, and other endcap propertiesand characteristics may be varied in accordance with a specifiedapplication and may or may not be the same as those of the optical fiber102.

It is appreciated that various endcap materials having no or low amountsof fluoride may be used in the present techniques. In one embodiment,the endcap material is made of an oxide-based glass material, such assilica (SiO₂), an aluminosilicate-based glass, a phosphosilicate-basedglass, an aluminophosphosilicate-based glass, a germanium-oxide-basedglass, a lead-germanate-based glass, a tellurium-oxide-based glass, andany combination thereof. In another embodiment, the endcap material ismade of another type of non-fluoride-based glass material, such as abarium gallium germanate (BGG) glass, a tellurite-based material, achalcogenide-based glass material, and any combination thereof. Inanother embodiment, the endcap material may be a non-glass-basedmaterial. Non-limiting examples include, to name a few, oxide-basedcrystal materials, such as sapphire (Al₂O₃), yttrium aluminum garnet(YAG; Y₃Al₅O₁₂), and other types of garnet crystal materials; and othertypes of crystal materials, such as zinc selenide (ZnSe) and diamond. Itis appreciated that these endcap compositions may be used for the endcap110 in FIG. 2 as well.

Examples & Experimentation

Various aspects of the present techniques were tested in a study, asreported in the following article, the entire contents of which areincorporated herein by reference: Y. O. Aydin, F. Maes, V. Fortin, S. T.Bah, R. Vallée, and M. Bernier, “Endcapping of high-power 3 μm fiberlasers,” Opt. Express 27(15), 20659-20669 (2019). It will be appreciatedfrom the overall description of this study that the fiber laser systemsand materials as well as the associated methods described herein mayhave a number of optional features, variations, and applications. Inparticular, the following description of experimentation and results isprovided to further illustrate some aspects of the disclosed principles,but should not be construed as in any way limiting their scope.

The present study investigated the efficiency of different endcapmaterials to mitigate fiber tip degradation of high-power 3-μm-classfluoride-based fiber lasers. For this purpose, endcaps made offluoride-based glass fibers (zirconium and aluminum fluorides),oxide-based glass fibers (lead-germanate and silica), and single-crystalsapphire fibers were spliced at the output of a high-power fiber laseroperating near 3 μm and monitored for degradation over a period of 100hours. The fluoride-based endcaps were found to undergo catastrophicfailure after less than 10 hours. The oxide and crystal-based endcapswere found able to withstand the 100-hour test, but underwent anonnegligible increase in temperature over time, suggesting that theymay be suitable to extend the lifetime of high-power fiber laser systems(e.g., 20-watt power level or more). The present study also proposedanother technique for suppressing OH diffusion within endcap material.The technique involved sputtering deposition of a thin film of siliconnitride (Si₃N₄) on the output face of the endcap to act as an OHdiffusion barrier. The technique was tested on ZrF₄, AlF₃, and Al₂O₃endcaps. The tested endcaps showed no sign of degradation underhigh-power 3-μm radiation over more than 100 hours of experimentation.

Experimental Setup

A home-made high-power 3-μm-class fiber laser system was used toinvestigate the degradation of the different endcap materials. The lasersystem is depicted schematically in FIG. 10 and is similar to the systemreported in [4]. Briefly, the laser system included a 6.5-m-long,double-clad, 7-mol %-erbium-doped fluorozirconate (Er³⁺:ZrF₄) fibermanufactured by Le Verre Fluoré. The 15-μm-diameter core of the fiberhad a numerical aperture of 0.12, enabling single-mode operation above2.4 μm. The fiber laser cavity was bounded by two intracore fiber Bragggratings (FBGs), which were written through the polymer jacket of thefiber using femtosecond pulses in accordance with a scanning phase-masktechnique [10, 11]. The entrance, high-reflectivity (HR) FBG had areflectivity higher than 99% at 2.83 μm, while the exit,low-reflectivity (LR) FBG had a reflectivity of 8% at 2.83 μm. Theactive fiber and the HR and LR-FBGs were spooled on a grooved andfan-cooled aluminum spool having a 32-cm diameter, and secured withUV-cured polymer.

The laser system was optically pumped from the forward end only, using a135-W commercial InGaAs 980-nm multimode laser diode. The laser diodehad a silica delivery fiber that was fusion-spliced (S1) to theEr³⁺:ZrF₄ fiber. At the output of the fiber laser cavity, a residualcladding pump stripper (RCPS) was fabricated by applying high-indexUV-cured polymer on the bare Er³⁺:ZrF₄ fiber. This pumping schemeenabled an efficiency of 23% with respect to the launched pump and apump-power-limited maximum output power of around 29 W at 2.83 μm. Asingle-mode fusion-splice (S2) was provided between the output Er³⁺:ZrF₄fiber and a mode-matched passive Er³⁺:ZrF₄ relay fiber to carry outmultiple endcap degradation experiments. The relay fiber had a 15-μmcore diameter, a numerical aperture of 0.12, and a 250-μm claddingdiameter. The high-power all-fiber laser cavity was operated at anoutput power of around 20 W for all degradation experiments.

The degradation over time of the various endcaps was monitored bymeasuring the temperature of their output faces with a thermal camera(Jenoptik, VarioCAM®) equipped with a close-up lens. Simultaneously, theoutput power of the laser system was recorded with a thermopile detector(Gentec-EO, UP25N-250E-H12-D0) to ensure the laser system operated at a20-W output power level throughout the experiments. It is noted that thelaser cavity was operated at this output power level with the samenominal performances for over 800 hours in the course of the presentstudy, with RMS fluctuations smaller than 0.1%.

Endcap Splicing and Manufacturing

The present study monitored the degradation of six different endcapmaterials:

-   -   fluorozirconate glass (ZrF₄—BaF₂—LaF₃—AlF₃—NaF—SrF₂—HfF₄),        fluoroaluminate glass (AlF₃—AlCl₃—NaF—ZrF₄—YF₃—SrF₂—BaF₂—LaF₂),        lead-germanate glass (GeO₂—ZnO—PbO—K₂O—PbF₂), silica glass        (SiO₂), and single-crystal sapphire (Al₂O₃). All endcap        materials were provided in fiber (or single-crystal fiber) form        and their specifications are presented in Table 1 below. The        silica fiber was home-drawn using a Heraeus preform composed of        a F-300 pure silica core and a F-320 fluorine-doped silica        cladding [15]. Manufacturing an endcap out of a 50%-doped        Er³⁺:YAG single crystal (Y₃Al₅O₁₂) fiber was also studied given        the unavailability of an undoped YAG fiber. It is noted that the        studied endcap materials will be referred to hereinbelow by        their main constituent, that is, ZrF₄ for fluorozirconate, AlF₃        for fluoroaluminate, and so on.

TABLE 1 Endcap Specifications. α^(c) T_(g) ^(d) Ø_(c) ^(f) L^(g) EndcapManufacturer n^(b) [×10⁻⁶K⁻¹] [° C.] [μm] [μm] ZrF₄ Le Verre Fluoré 1.4917.2 265 200 480 AlF₃ Fiberlabs 1.46 18.6 390 200 450 GeO₂ InfraredFiber Systems 1.83 10.9 420 230 380 GeO₂ Le Verre Fluoré 1.83 10.9 420230 410 SiO₂ Heraeus F-300 1.42 0.55 1175 242 190 Er³⁺: Shasta Crystals1.79 6.14 T_(f) = 220 320 YAG 1940^(e) Al₂O₃ Shasta Crystals 1.72 5-5.6T_(f) = 240 N.A. 2030^(e) ^(a)Optomechanical properties taken from[12-15]. ^(b)Refractive index around 3 urn. ^(c)Thermal expansioncoefficient. ^(d)Transition temperature. ^(e)Melting temperature.^(f)Core diameter. ^(g)Length.

The endcaps were fusion-spliced to the passive ZrF₄ relay fiber using aVytran® GPX system equipped with an iridium filament (Vytran®, FRAV4).For the ZrF₄ endcap, the filament was positioned at the splice pointbetween the relay fiber and the ZrF₄ endcap. All other endcaps werespliced to the ZrF₄ relay fiber by offsetting the longitudinal positionof the filament toward the endcap material, as detailed in [16]. Oncethe fusion splice was achieved, the endcap material was cleaved at agiven length with a Vytran® LDC cleaver. Images of the final endcapsresulting from this manufacturing process are presented in FIGS. 12(a)to 12(c). Typical output power losses at 2.83 μm after splicing wereabout 4% for the fluoride endcaps (i.e., the ZrF₄ and AlF₃ endcaps) andabout 8% for the GeO₂ endcaps, including fiber transmission losses andFresnel reflections at the output endface of the endcap and at thesplice interface. Prior to conducting the degradation tests, theassembly was secured using a low-index UV-cured polymer in a copperV-groove to ensure good heat conduction from the output endface to aheat sink. Care was taken to limit the length of the endcap protrudingout of the copper V-groove. Thermal conduction was chosen in the presentstudy to limit the maximum endface temperature and the rate of the OHdiffusion process.

In contrast to the fluoride and the GeO₂ endcaps, the SiO₂, Er³⁺:YAG,and Al₂O₃ endcaps formed no permanent thermal bonds when spliced to theZrF₄ relay glass fiber. To overcome this limitation, the larger thermalexpansion coefficient of ZrF₄ relative to the tested oxide-based endcapmaterials (see Table 1) was relied upon. By pushing in a controlledmanner, the oxide-based endcap into the ZrF₄ relay fiber, after theendcap had been heated sufficiently, a permanent and robust joint wasobtained, as seen in FIGS. 12(d) to 12(f). The strength of the joint wasprovided mainly by the ZrF₄ glass, which wrapped tightly around theoxide-based endcap after cooling down of the splice point. It is notedthat all fusion-splices resulting from this splicing procedure wereproof-tested at a tension of 200 g (roughly 4.4 MPa) prior to being usedin the tests. The typical output power losses at 2.83 μm after splicingthe SiO₂, Er³⁺:YAG and Al₂O₃ endcaps were 8%, 10%, and 16%,respectively.

In the case of the SiO₂ and the Er³⁺:YAG endcaps, it was possible tocleave the endcap material after the splicing process, as seen in FIGS.12(d) and 12(e), and cool the assembly in the same manner as thefluoride and the GeO₂ endcaps. It is noted that the length of the SiO₂endcap was shortened as much as possible due to its high absorptionlosses of about 25 dB/m near 2.83 μm. For the Er³⁺:YAG endcap, cleavingwas simplified by the fact that the crystalline planes of the Er³⁺:YAGfiber are perpendicular to its optical axis, as shown in [17].Photographs of the splice interfaces between the ZrF₄ relay fiber andthe SiO₂ and the Er³⁺:YAG endcaps are shown in FIGS. 13(a) and 13(b),respectively. From FIG. 13(a), it is appreciated that the interfacebetween the SiO₂ endcap and the ZrF₄ relay fiber was smooth and that itdid not deteriorate the quality of the laser beam. As for the Er³⁺:YAGendcap, one can see from FIG. 13(b) some bubbles at the interface, whichcould degrade the beam quality if provided in the beam path. However, itis believed that the splicing recipe could be modified to prevent theformation of such bubbles and enable cleaner splice interfaces, similarto the one depicted in FIG. 13(a) for the SiO₂ endcap. For thesingle-crystal sapphire fiber, it was not possible to cleave or polishthe endcap without breaking the splice point, due to the crystallineplanes being oriented at 45° with respect to the fiber axis and to thehigh mechanical strength of the endcap material. Therefore, the entirelength of the sapphire fiber (i.e., about 50 cm) was kept for thedegradation test. Furthermore, given that the sapphire fiber wascoreless, attempts at cooling the fiber extremity using the copperassembly described above resulted in leakage of the 3-μm signal from theside and the eventual failure of the assembly. Therefore, the sapphirefiber tip was tested under natural heat convection, instead of heatconduction as for the other endcaps.

Results and Discussion

Endcap Degradation.

FIG. 14 illustrates the degradation of the endcaps under the action of20-W CW output power at 2.83 μm over a 100-hour time period. The initialtemperature of the different endcaps was found to vary between about 40°C. and 75° C., a variation accounted for by differences in endcapparameters, such as the initial OH-compound concentration, theabsorption coefficient at 2.83 μm, the thermal conductivity, and therefractive index (the refractive index determining the intensity ofFresnel reflections at the endcap interface). As reported in [4], thefluoride-based endcaps did not withstand the experiment more than 10hours. While the initial temperature of the multimode ZrF₄ endcap wasthe lowest of all the endcaps that were tested (40° C.), it underwentcatastrophic failure after only 10 minutes. The degradation curve of theZrF₄ endcap, as well as the time elapsed before failure, agree with theresults reported in [9]. The AlF₃ endcap survived for about 10 hoursunder similar conditions, reflecting the fact that its glass matrix ismore than one order of magnitude more stable in water than that of ZrF₄[18]. Thus, the present study found that AlF₃-based endcaps—and evenmore so ZrF₄-based endcaps—generally provide unsuitable, long-termsolutions when operating around 3 μm with output powers greater than afew watts. Based on previous reports having used AlF₃ endcaps to protectfiber laser systems from photodegradation [19, 20], it may be concludedthat the use of AlF₃ endcaps to achieve long-term operation isappropriate for 3-μm fiber laser systems operating with output powers ofat most a few watts.

From FIG. 14, it can be seen that the oxide-based and crystallineendcaps that were tested all survived the 100-hour-long degradationexperiment. However, the experiment also revealed that their temperatureincreased over time, thus indicating the existence of some degradationat the output power of 20 W. Table 2 below summarizes the degradationperformance of the SiO₂ and the GeO₂-based endcaps. The initialtemperature of the SiO₂ endcap (74° C.) was noticeably higher than thatof the two GeO₂ endcaps (44° C. and 53° C.), a direct result of thestrong absorption of SiO₂ around 3 μm. Given the constant ambienttemperature during the experiment of 20° C., the initial temperaturerise of the SiO₂ endcap per watt of output power at 3 μm was found to be2.70° C./W, while those of the two GeO₂ endcaps were about twice less,that is, 1.20° C./W and 1.40° C./W. Hence, in the perspective ofpower-scaling the output power of 3-μm-class all-fiber lasers to 100 W,one can expect an SiO₂ endcap to reach an initial temperature of about290° C. and a GeO₂ endcap to reach an initial temperature rangingbetween about 140° C. and 218° C. Thus, GeO₂ endcaps were foundgenerally to be better candidates for high-power 3-μm systems since thesplice between the SiO₂ endcaps and the ZrF₄ fiber generally cannotwithstand temperatures in excess of the transition temperature of ZrF₄(i.e., about 270° C., see [12]). Nonetheless, for high-power systems(e.g., operating at about 20 W), SiO₂ endcaps may provide a moresuitable alternative than GeO₂ endcaps, given that the degradation rateof the former is more than three times slower than that of the latter.This smaller degradation rate enabled the SiO₂ endcap to reach, after100 hours, a final temperature similar to that of the GeO₂ endcap fromLe Verre Fluoré, although the initial temperature of the former was 33°C. higher than that of the latter. Furthermore, SiO₂ endcaps aregenerally less expensive and easier to handle and process than GeO₂endcaps. Additionally, the refractive index of SiO₂ around 3 μm (1.42)is closer to the refractive index of ZrF₄ glass (1.49) than is therefractive index of GeO₂ (1.83). This characteristic may also favor SiO₂endcaps in the design of powerful mid-infrared mode-locked orin-amplifier supercontinuum fiber lasers [21, 22], as mentioned above.

TABLE 2 SiO₂ and GeO₂ endcap performances. T_(i) ^(a) ΔT_(i)/ΔP^(b)T_(i, 1000 W) ^(c) ΔT/Δt^(d) Endcap Manufacturer [° C.] [° C./W] [° C.][° C./h] GeO₂ Le Verre Fluoré 44 1.20 140 0.37 GeO₂ Infrared Fiber 531.65 218 0.47 Systems SiO₂ Heraeus F-300 74 2.70 290 0.10 ^(a)Initialtemperature. ^(b)Ti variation with 3-μm output power. ^(c)ExtrapolatedTi at 100 W output power. ^(d)Temperature variation over time.

As for the SiO₂ and GeO₂ endcaps, the Al₂O₃ fiber tip exhibited signs ofdegradation over time under the influence of the high-power 3-μm laserlight. The initial temperature of the Al₂O₃ fiber tip was 60° C. and itsfinal temperature was 97° C., corresponding to a degradation rate of0.37° C./h. It is noted that the initial temperature and degradationrate of the Al₂O₃ fiber tip were comparable to those of the GeO₂endcaps. However, the Al₂O₃ fiber tip experienced natural convection,rather than heat conduction as for the other endcaps, which mayaccelerate degradation by an order of magnitude [9]. Therefore, it isenvisioned that the use of Al₂O₃ endcaps, rather than fiber tips, mayprovide a solution to the issue of photodegradation caused by moisturediffusion in high-power 3-μm fiber laser systems, contingent upon theability of manufacturing endcaps from single-crystal Al₂O₃ fibers. Onealternative to manufacturing Al₂O₃ endcaps would be to inscribedepressed-cladding single-mode waveguides with femtosecond pulses in theAl₂O₃ rod fiber, as reported recently in [23]. Such an approach couldpreserve the beam quality of the 3-μm fiber laser system despite thelong lengths of Al₂O₃ fiber used for beam delivery purposes.

The Er³⁺:YAG endcap could not be tested since its temperature wasalready around 120° C. at 3 μm and a power level of 2.4 W. Nonetheless,it is envisioned that undoped single-crystal YAG fibers could provideefficient endcap materials because they possess thermal and mechanicalproperties similar to those of Al₂O₃ fibers and may be readily processedinto endcaps at the tip of ZrF₄ fibers.

Si₃N₄ Coating for High-Power 3-μm Fiber Lasers.

In order to further inhibit OH diffusion within endcap materials, theoutput face of a number of endcaps were coated with a nanoscopic thinfilm of silicon nitride (Si₃N₄). Silicon nitride is commonly used inelectronics as a diffusion barrier for SiO₂ dielectric layers or aspassivation layers in flexible electroluminescent devices.

In the present study, the Si₃N₄ thin films were deposited on the outputface of the endcaps using reactive ion beam-assisted double magnetronsputtering, such as described in [24], under a 1.46×10⁻³ torrenvironment. The target material was a six-inch-diameter, 99.99 percentpure silicon disk. The temperature of the substrate was kept at 115° C.Deposition of the thin film was done at a 0.24 nm/s deposition rate. Anargon gas flow was maintained at 32 SCCM during the sputtering process,and a reactive gas (nitrogen; 22 SCCM) was introduced inside the chamberby the ion source. The uniformity of the deposited thin film wasenhanced by rotating the substrate holder at 80 rpm.

FIGS. 15(a) to 15(c) compares the degradation of Si₃N₄-coated anduncoated ZrF₄ [FIG. 15(a)], AlF₃ [FIG. 15(b)], and Al₂O₃ [FIG. 15(c)]endcaps under the impact of 3-μm light over a 100-hour time period. Thethickness of the Si₃N₄ coating was a 25 nm for the ZrF₄ and AlF₃ endcapsand 100 nm for the Al₂O₃ endcaps. For both ZrF₄ and AlF₃, the coatingthickness was limited to less than one percent of the wavelength tolimit Fresnel reflections because of the high refractive index (n˜1.95)of Si₃N₄. The output power of the 3-μm fiber laser was 7 W for the ZrF₄and AlF₃ endcaps and 20 W for the Al₂O₃ endcaps. All endcaps experiencednatural convection to accelerate the photodegradation process. FromFIGS. 15(a) to 15(c), it is seen that the provision of the Si₃N₄ coatinginhibited OH diffusion for all the tested endcaps, as no increase intemperature over time was recorded. FIGS. 15(a) to 15(c) also illustratethat Si₃N₄ coatings may be applied on a variety of fiber-opticmaterials.

CONCLUSION

In the present study, the OH degradation of various fiber endcapsspliced at the output of a 20-W all-fiber laser operating at 3 μm wasmonitored over a 100-hour time period. The present study found that thefluoride-based endcaps (i.e., the ZrF₄- and AlF₃-based endcaps) lastedfor less than 10 hours before undergoing catastrophic failure. The studyalso found that the oxide-based endcaps (i.e., the GeO₂- and SiO₂-basedendcaps) and the Al₂O₃ fiber tip survived the experiment, thus makingthem attractive endcap solutions in some applications, for example, atoutput power levels between about 1 and 20 W. In order to further reduceOH diffusion with endcap materials under the irradiation of intense 3-μmlight, the study also proposed to coat the output face of endcaps with asilicon nitride (Si₃N₄) thin film. The potential of the proposedapproach in reducing OH diffusion was tested on a ZrF₄ endcap, an AlF₃endcap, and an Al₂O₃ fiber tip, each of which coated with a Si₃Na thinfilm. Upon illumination with 3-μm light over a 100-hour time period, thecoated endcaps and fiber tip showed no sign of degradation, whereastheir uncoated counterparts either underwent catastrophic failure (ZrF₄and AlF₃) or showed a nonnegligible temperature rise (Al₂O₃).

Numerous modifications could be made to the embodiments described abovewithout departing from the scope of the appended claims.

REFERENCES

The following is a list of references, the entire contents of which areincorporated herein by reference.

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1. An optical fiber product, comprising: an optical fiber made of afiber-optic material and configured for propagation of mid-infraredradiation toward a light-radiating endface of or coupled to the opticalfiber; and a diffusion barrier disposed on the light-radiating endfaceand configured for allowing the mid-infrared radiation emanating fromthe light-radiating endface to pass therethrough and for preventing OHdiffusion therethrough toward the light-radiating endface.
 2. Theoptical fiber product of claim 1, wherein the fiber-optic materialcomprises a glass material.
 3. The optical fiber product of claim 2,wherein the glass material comprises a fluoride-based glass, achalcogenide-based glass, a chalcohalide-based glass, an oxide-basedglass, a tellurite-based glass, or any combination thereof.
 4. Theoptical fiber product of claim 1, wherein the fiber-optic materialcomprises a crystal material.
 5. The optical fiber product of claim 1,wherein the light-radiating endface is an endface of the optical fiber.6. The optical fiber product of claim 1, wherein the light-radiatingendface is an endface of an endcap coupled between the optical fiber andthe diffusion barrier.
 7. The optical fiber product of claim 6, whereinthe endcap is made of an endcap material that is less permeable to OHdiffusion than the fiber-optic material.
 8. The optical fiber product ofclaim 1, wherein the light-radiating endface is perpendicular to alongitudinal fiber axis of the optical fiber product.
 9. The opticalfiber product of claim 1, wherein the light-radiating endface is obliqueto a longitudinal fiber axis of the optical fiber product.
 10. Theoptical fiber product of claim 1, wherein the diffusion barriercomprises a thin-film coating.
 11. The optical fiber product of claim10, wherein the thin-film coating comprises a single thin-film layer.12. The optical fiber product of claim 10, wherein the thin-film coatingcomprises multiple thin-film layers.
 13. The optical fiber product ofclaim 1, wherein the diffusion barrier extends over all of thelight-radiating endface.
 14. The optical fiber product of claim 1,wherein the diffusion barrier has a thickness ranging from about 1 nm toabout 10 μm.
 15. The optical fiber product of claim 14, wherein thethickness of the diffusion barrier ranges from about 10 nm and about 100nm.
 16. The optical fiber product of claim 1, wherein the diffusionbarrier is made of a barrier material comprising a dielectric, acarbon-based material, a metal, a metalloid, a metal oxide, an alloy, acomposite material, or any combination thereof.
 17. The optical fiberproduct of claim 1, wherein the diffusion barrier is made of a barriermaterial comprising a ceramic compound.
 18. The optical fiber product ofclaim 16, wherein the ceramic compound comprises a carbide, a nitride, aboride, an oxide, or any combination thereof.
 19. The optical fiberproduct of claim 16, wherein the ceramic compound comprises siliconnitride, silicon oxynitride, silicon carbide, boron nitride, siliconcarbide, boron carbide, tungsten carbide, or any combination thereof.20. A fiber-based laser oscillator, laser amplifier, or laser beamdelivery system comprising the optical fiber product of claim
 1. 21. Anoptical fiber product, comprising: an optical fiber made of afiber-optic material and configured for propagation of mid-infraredradiation; and an endcap having a proximal endface coupled to theoptical fiber for receiving therefrom the mid-infrared radiation, adistal endface for radiating the mid-infrared radiation outside theoptical fiber product, and an endcap body extending and configured forpropagation of the mid-infrared radiation from the proximal endface tothe distal endface, the endcap being made of an endcap material that hasno or a low amount of fluoride and that is less permeable to OHdiffusion than the fiber-optic material.
 22. The optical fiber productof claim 21, wherein the fiber-optic material comprises a glassmaterial.
 23. The optical fiber product of claim 22, wherein the glassmaterial comprises a fluoride-based glass, a chalcogenide-based glass, achalcohalide-based glass, an oxide-based glass, a tellurite-based glass,or any combination thereof.
 24. The optical fiber product of claim 21,wherein the fiber-optic material comprises a crystal material.
 25. Theoptical fiber product of claim 21, wherein the endcap material comprisesa glass material.
 26. The optical fiber product of claim 25, wherein theglass material comprises an oxide-based glass.
 27. The optical fiberproduct of claim 26, wherein the oxide-based glass comprises silica, analuminosilicate-based glass, a phosphosilicate-based glass, analuminophosphosilicate-based glass, a germanium-oxide-based glass, alead-germanate-based glass, a tellurium-oxide-based glass, or anycombination thereof.
 28. The optical fiber product of claim 25, whereinthe glass material comprises a barium gallium germanate glass, atellurite-based glass, a chalcogenide-based glass, or any combinationthereof.
 29. The optical fiber product of claim 21, wherein the endcapmaterial comprises a crystal material.
 30. The optical fiber product ofclaim 29, wherein the crystal material comprises an oxide-based crystalmaterial.
 31. The optical fiber product of claim 30, wherein theoxide-based crystal comprises sapphire, a garnet crystal material, or acombination thereof.
 32. The optical fiber product of claim 21, whereinthe endcap material has a molar proportion of fluoride or fluoride-basedcompounds that is less than 20 mol %.
 33. The optical fiber product ofclaim 32, wherein the molar proportion of fluoride or fluoride-basedcompounds of the endcap material is less than 10 mol %.
 34. The opticalfiber product of claim 33, wherein the endcap material contains no ortrace amounts of fluoride or fluoride-based compounds.
 35. The opticalfiber product of claim 21, wherein the endcap has a length ranging fromabout 50 μm to about 6 mm.
 36. The optical fiber product of claim 21,wherein the endcap has a coreless structure.
 37. The optical fiberproduct of claim 21, wherein the endcap has a core-clad structure. 38.The optical fiber product of claim 21, wherein the distal endface of theendcap is perpendicular to a longitudinal fiber axis of the opticalfiber product.
 39. The optical fiber product of claim 21, wherein thedistal endface of the endcap is oblique to a longitudinal fiber axis ofthe optical fiber product.
 40. A fiber-based laser oscillator, laseramplifier, or laser beam delivery system comprising the optical fiberproduct of claim 21.